1. Field of the Invention
The present invention relates to the safety of employees in elevated work positions and, more specifically, to vertical lifeline systems.
2. Related Art
An overhead bridge crane contained in a building runs on elevated beams or rails. There are two parallel beams or rails called runway beams. Perpendicular to the runway beams is the bridge or girder (also called the “crane”). The bridge, or “crane,” is connected to the runway beams by two end trucks on each end of the bridge. The end trucks can be anywhere from five feet long for a small crane to nearly twenty feet long for a long span crane. The bridge can move in either direction along the runway beams (“North-South” will be used herein to describe this “bridge travel”) via these end trucks. On the bridge is a trolley, which can move in either direction along the bridge (“East-West”). The trolley holds the working hoist, which can move Up or down (“Up-Down”); the working hoist in a bridge crane lifts and lowers the object of the work, the load. Most bridge cranes are controlled via remote control radio transmitters. The structure of bridge cranes allows 3-axis motion in the X, Y, and Z planes and thus allows full coverage of the floor to perform work.
When a crane operator doing work with an overhead bridge crane (a “working crane”) is more than six or seven feet above the floor, the operator is required by Occupational Health and Safety Administration (OSHA) to use fall protection equipment. Since a considerable amount of crane work is elevated, or “up,” elevated crane operators, or “fall operators,” generally wear fall protection equipment, such as a personal fall arrest system. However, many fall operators use no protection, which is a direct violation of OSHA. When protection is used, methods are often cumbersome and restrictive, and are generally a hassle.
In one type of personal fall arrest system, a Vertical Lifeline System, a fall operator is attached to an anchorage overhead. The system consists of a fixed anchorage, a self-retracting lifeline (SRL), and a body harness. When the fall operator moves, the fixed anchorage is no longer directly above the fall operator, creating an angle. This leads to a pendulum effect if a fall operator should fall, which is not ideal under the American National Standards Institute (ANSI) codes regarding verticality of fall restrictions.
A second fall arrest system, the Monorail Beam, consists of a beam located under the runway beam that swings out parallel to the runway. This method is problematic because the beam is usually located where the crane needs to work. In addition, the SRLs used move trolleys with considerable rolling resistance. Consequently, when a fall operator needs to move, that fall operator has to tug at the SRL to move the trolley along the beam, which requires significant effort. Moreover, the Monorail Beam system is quite expensive and often requires a hydraulic power supply to move it.
A third and more commonly used fall arrest system is comprised of a second overhead bridge crane (a second bridge with a light capacity), or “fall crane.” This fall crane is placed on the same runway beams as the working crane. Thus, the working crane and the fall crane are parallel and adjacent to each other. An anti-collision system on the working crane is needed so that the working crane and the fall crane do not accidentally collide. One or two trolleys are placed on the fall crane bridge; an SRL is connected to each trolley, and a body harness is attached to each SRL. By installing either one or two trolleys, the fall crane can be designed to support either one or two fall operators. There are various problems with this fall protection method, as described below:
a) It is not cost-efficient to buy and maintain a second crane (the fall crane) that does no work. Additionally, it is less efficient and more costly to have two cranes in operation (the working crane and the fall crane) instead of one.
b) Two transmitters are needed, one for the working crane and a second for the fall crane. This results in two transmitters to maintain and control instead of one.
c) Since the fall crane is always on one side or the other of the working crane, the fall crane is often in the way of the working crane. If the working crane needs to be moved, either the fall crane must first be moved out of the way, or the fall crane must be moved along with the working crane. This extra step is inefficient and may require a second crane operator in addition to the operator for the working crane.
d) The working crane and the fall crane often need to be positioned close to each other to get work done. In order to position the two cranes close to one another, an anti-collision override on the working crane must be activated. This requires the pushing of two buttons instead of one: the anti-collision override button and the normal move button.
e) Since the fall crane is always on one side or the other of the working crane, the two cranes are always separated; the greater the length of the end trucks, the greater the separation between the two cranes. As a result of this separation, a fall operator positioned by the working crane is attached to a lifeline that is not directly vertical. The angle that is created leads to a pendulum effect if a fall occurs, which is not ideal under the ANSI codes regarding verticality of fall restrictions.
f) If the fall crane is designed to support two fall operators (if there are two SRLs attached to the fall crane), the fall operators must work side by side on the fall crane and cannot “pass” each other (East-West) along the fall crane to work in each other's location. This restricts a fall operator's ability to get work done. The side-by-side positioning of two fall operators also creates an angle problem, leading to a pendulum effect if a fall occurs.
g) The trolleys have significant rolling resistance. As a result, when a fall operator wants to move East-West, that operator has to tug at the SRL to move the trolley along the bridge, which requires significant effort.
h) The working crane has unrestricted movement at all speeds. As a result, a fall operator can unintentionally drag himself (and another fall operator if there are two) off of the equipment.
i) When there are two operators “up,” there is the risk that one of the fall operators will not be in a position where he is ready for the other fall operator to move the crane.
For the foregoing reasons, the second overhead bridge crane method is not ideal.